Femtosecond Heterodyne Transient-Grating Studies of Nonradiative Decay of the S2 (11Bu+) State of 尾-Carotene: Contributions from Dark Intermediates and Double-Quantum Coherences

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• 作者：Soumen Ghosh ; Michael M. Bishop ; Jerome D. Roscioli ; Jenny Jo Mueller ; Nolan C. Shepherd ; Amy M. LaFountain ; Harry A. Frank ; Warren F. Beck
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：November 25, 2015
• 年：2015
• 卷：119
• 期：47
• 页码：14905-14924
• 全文大小：923K
• ISSN：1520-5207

文摘

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in 尾-carotene from the S₂ (1¹B_u⁺) state to the S₁ (2¹A_g^{鈥?/sup>) state in benzonitrile solution. The results indicate definitively that the S₂ state populates an intermediate state, S_x, on an ultrafast time scale prior to nonradiative decay to the S₁ state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the S_x state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S₂ state. The finding that the excited-state absorption spectrum from the S_x state is significantly red-shifted from that of S₂ and S₁ leads to a new assignment for the spectroscopic origin of the S_x state. Rather than assigning S_x to a discrete electronic state, such as the 1B_u鈥?/sup> state suggested in previous work, it is proposed that the S_x state corresponds to a transition-state-like structure on the S₂ potential surface. In this hypothesis, the 12 fs time constant for the decay of the S₂ state corresponds to a vibrational displacement of the C鈥擟 and C鈺怌 bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck鈥揅ondon structure to a potential energy barrier on the S₂ surface that divides planar and torsionally displaced structures. The lifetime of the S_x state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S₁ state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S₂ torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.}